Magnetic structure for traveling wave tubes



May 7, 1957 M. s. GLASS MAGNETIC STRUCTURE FOR TRAVELING WAVE TUBESFiled April 25, 1956 2 Sheets-Sheet l lNVENTOR M. 5. GLASS ATTORNEY y 7,1957 M. s. GLASS 2,791,718

MAGNETIC STRUCTURE FOR TRAVELING WAVE TUBES Filed April 23, 1956 2Sheets-Sheet 2 I I I 1 IIIIIIIIIIII I lNl ENTOR M. S. GLASS ATTORNEYUnited States Patent 1 2,791,718 MAGNETIC STRUCTURE FOR TRAVELING WAVETUBES Myron S. Glass, West Orange, N. J., assignor to Bell TelephoneLaboratories, Incorporated, New York, N. Y.,

a corporation of New York Application April 23, 1956, Serial No. 579,91215 Claims. (Cl. 315-3.5)

This invention relates to apparatus including magnetic structures, andmore particularly to such apparatus including traveling wave tubeswherein an electron beam is focused by a magnetic field along arelatively long path.

in certain electron discharge devices, such as traveling wave tubes, anelectron stream is projected into an interaction space generally definedby a helix, where it is made to interact with an electromagnetic wavetraveling along the helix. Optimum operation is achieved when theelectron stream is confined to a substantially cylindrical form havingelectrons at its radial extremities close to but not impinging thehelix: throughout the interaction space. It has been the practice toestablish a longitudinal magnetic field along the path of electron flowto confine the beam as desired.

it is an object of this invention to improve magnetic focusing ofelectron beams in electron discharge devices such as traveling wavetubes.

More specifically, it is an object of this invention to provide amagnetic field which will focus an electron stream over a relativelylong path, confining the stream to a uniform shape within narrow limits.

Another object of this invention is to provide a uniform straightmagnetic field of sufficient strength to achieve the desired focusing ofan electron beam while reducing the size and weight of the focusingequipment.

in electron discharge devices employing high density electron beams,such as traveling wave tubes, a longitudinal field is encountered in adrift space of the device, and electrons entering this field attain anangular velocity proportional to the difference in magnetic fluxencountered in going from a shielded electron gun region into the fieldregion. The inward or focusing force per charge is proportional to theproduct of the angular velocity the longitudinal magnetic field oreffectively the square of the magnetic field. This inward force isadjusted to counterbalance exactly the sum of outward mutually repulsiveforces of the electrons, generally described as space charge forces, andthe outward centrifugal force of the spiraling electrons.

In the past, in order to provide a uniform straight longitudinalmagnetic field of sufiicient strength to offset the large space chargeforces existing in an electron stream of high density over a relativelylong electron path, the permanent magnets required were many times theweight and size of the device employing my invention. From thestandpoints of expense, compactness and portability, among others, it isdesirable to reduce the size and weight of the focusing equipment.

in order to attain the precise degree of focusing desired without theneed for employing large and cumbersome magnets, there has beensuggested a system of periodic focusing to overcome the dliTlCllltlGSencountered in straight field focusing. In such a system, a series ofpole pieces having successively opposite polarity deployed along thepath of electron flow to establish a succession of axially symmetricregions of longitudina magnetic field. Concentration of the field insuch a succession of relatively short regions rather than a uniformfield over a relatively long region, as in conventional straight fieldpermanent magnets, permits a reduction in total magnet size and weightwhile maintain- 2,791,718 Patented May 7, 1957 ing the desired fielddistribution. An example of such a system is disclosed in an articleElectron Beam Focusing with Periodic Permanent Magnet Fields by I. T.Mendel, C. F. Quate and W. H. Yocom, Proceedings of the I. R. E., volume42, pages 800-810, May 1954.

A proposal by P. P. Ciofii, described in his application Serial No.543,235, filed October 27, 1955, suggests employment of a singlepermanent magnet extending over the length of the interaction space ofthe traveling wave tube and having an axial cylindrical hole through themagnetic material concentric with the longitudinal axis of the magnet,in which axial hole the helical conductor of the traveling wave tube ispositioned. The required volume of uniform air gap field can bedeveloped within this hole. It is customary in traveling wave tubeapplications to provide coaxial or wave guide entrance and exit pointsto the interaction space. A small hole adjacent each end of thepermanent magnet penetrating to the axial interaction space allows forcoaxial fittings without disrupting the continuity of the magneticcircuit. Larger openings in the magnetic material are required for waveguide fittings, however, and resultant discontinuities may affect themagnetic circuit appreciably.

The straight field air gap volume may be reduced in dimensions, inaccordance with my invention, to those obtainable with periodic fieldstructures for particular focusing requirements and the straight fieldmagnet weight and volume made comparable to that of the periodic fieldstructure. This is accomplished with adequate provision for wave guidecoupling and presents a rugged, easily supported assembly. Thus thebenefits of a straight and substantially uniform magnetic field can beachieved in wave guide coupled traveling Wave tube applications withoutthe encumbering weight and volume of prior straight magnetic fielddevices.

In addition, a marked reduction in undesirable transverse magnetic fieldcomponents and adequate shielding against external fields is realized soas to permit the use of field straightening apparatus known in the art,with optimum results.

My invention accomplishes these results by employing a pair of permanentmagnets extending over the length of the interaction space of thetraveling wave tube and positioned on opposite sides thereof. Each ofthe magnets comprises a flat bed portion having a pair of projections,protuberances, or cars, one extending from each side of the bed portionin a plane normal to the plane of the bed portion and tapered from anapex in the midsection toward the ends of the bed portion and toward themedian of its width. The apex area advantageously may be flattened toprovide a good contact surface to meet the corresponding extension ofthe other bed portion. When properly positioned the magnetssubstantially enclose the interaction space, the projections from thefiat bed portions of each magnet meeting the corresponding projectionsfrom the other magnet at the midsection. The taper in the projectionsprovides areas of access at the sides of the structure adjacent the endsto accommodate wave guide couplings.

I have found that the desired axial field distribution may be obtainedif the magnetic material cross-sectional area at the ends of theinteraction space of the traveling wave tube is about 60 percent of thatin the midsection. The tapered magnetic structure of the specificillustrative embodiment of this invention satisfies this requirement ofdistribution of cross-sectional area of magnetic material to obtain thedesired field. In traveling wave tube applications employing wave guidecouplings, it may be desirable to attain a uniform magnetic fieldintensity of 500 oersteds, for example. A suitable permanent magnetmaterial for straight magnetic fields of 500 oersteds is Alnico V, aniron alloy containing aluminum, nickel, cobalt,

"ice

copper and titanium, since it develops its optimum energy product at apoint having coordinates of flux density and field intensity of 8600gauss and 500 oersteds, respectively. I have found that in the specificillustrative embodiment of this invention wherein the permanent magnetis Alnico V and is given the tapered configuration describedhereinbefore, for a magnet length of 5.75 inches along the axis of theconductor and an inside diameter at the median section is 1.6 inches,the resultant volume of magnetic material to produce the desiredstraight magnetic field in the interaction space is comparable to thetotal required volume of periodic field magnetic materials. The weight,of course, is reduced proportionately. It is a feature of this inventionthat traveling wave tube apparatus comprise a pair of flat permanentmagnet members positioned on opposite sides of the tubes interactionspace, each member having projections or ears extending from its sidesto meet the projections from the opposite member and increasing inlength and width away from the areas of contact with each other andtoward the flat members.

It is another feature of this invention that the decrease of theprojections or ears at distances successively greater from the center ofthe magnetic structure is so proportioned as to give a suitable taper incross section area of magnetic material to attain a uniform or flatmagnetic field distribution along the axis of the magnetic structure.

Thus in accordance with this feature of the invention abruptdiscontinuities give rise to sharp changes in field strength on the axisand frequently to undesired crossfields.

It is a further feature of this invention that the protuberances or earsextending from the flat or bed portions of the magnetic members at themiddle close the gap at the sides of the magnet and provide eflt'ectiveshielding against disturbances from other magnets or magnetic materialsin the vicinity. Further shielding, in accordance with an aspect of thisinvention, may be provided by field straighteners around the travelingwave tube itself within the magnetic structure.

It is still a further feature of this invention that the ears be sotapered and dimensioned as to provide adequate clearance for themicrowave input and output circuits at the ends of the magneticstructure.

A complete understanding of this invention and of the various featuresthereof may be gained from consideration of the following detaileddescription and the accompanying drawing, in which:

Fig. 1 is a perspective view of one section of a permanent magnetstructure utilized in one specific embodiment of this invention;

Fig. 2 is a perspective view of a complete assembly utilizing sectionsas shown in Fig. 1;

Fig. 3 is a perspective view of one section of a variation of thepermanent magnet structure of Fig. 1;

Fig. 4 is a side view partly in section of apparatus utilized in onespecific illustrative embodiment of this invention employing the magnetstructure of Fig. 1; and

Fig.v 5 is a side'view in section showing additional apparatus inaccordance with another specific embodiment of this invention.

The specific illustrative embodiment of this invention as depicted inFig. 4 comprises a permanent magnet assembly best seen in theperspective views of Figs. 1 and 2. As seen in Fig. 2 the permanentmagnet assembly 10 comprises a lower section having a bed portion 12ofsubstantially parallelepiped configuration and tapered protuberancesor ears 13 extending from sides of the bed portion 12 in a plane normalto the plane of the flat inner surface 16 of the bed portion, The areaof greatest extension of the'ears 13'from the bed portion 12 is at themedian of the bed length, and each ear 13 has a flattened referencesurface 14 in this area. The ears 13 taper from this reference surfacetoward each end of the bed portion 12 and also toward the median of thebed width and join the opposing car 13 below the inner surface 16 of thebed portion 12. An upper section 15 of the magnet assembly 10 is of thesame configuration as the lower section 15. When the two sections 15 arejoined at the reference surfaces 14, a unitary assembly is presentedproviding an aperture therethrough which affords ade quate space for atraveling wave tube assembly and sufficient openings in the taperedsides for insertion of wave guide couplings for such an assembly.

I have found that tapering the cars 13 toward the median of the bedwidth so as to meet the median below the plane of the inner surface 16of the bed portions 12, as in Figs. 1 and 2, affords optimum results,with a minimum weight, although acceptable results are also obtained byemploying tapered sides of the cars 13 meeting the median of the bedwidth at or above the plane of the bed portion inner surface 16, as inFig. 3.

The traveling wave tube 20, which may be of any type known in the art,is inserted into the space between the upper and lower sections 11 and15 of the permanent magnet assembly 10 and extends over the length ofthe assembly 10. The traveling wave tube essentially comprises anelectron gun assembly 21, a helix transmission circuit 22, and anelectron collector assembly 23.

Input and output wave guides 25 and 26 are positioned within the endsections of the magnet assembly 10 transverse to the axis of thetraveling wave tube 20 in energy coupling relation with the input andoutput ends of the helix transmission circuit 22. The taper of theextension ears 13 toward each end of the magnet assembly 10 affordsready access for the wave guides and 26.

. The helix 22 extends within the magnet assembly 10 from the input waveguide 25 to the output wave guide 26. Pole pieces 28 and 29advantageously abut directly against the ends of the magnet bed portions12 of magnet assembly 10 and enclose the space between the ends of theupper and lower bed port-ions 12. The pole piece 28 has an aperturetherethrough of sufficient size to accommodate the enlarged portion ofthe envelope of the traveling wave tube 20, which enlarged portionencompasses the electron gun 21. Similarly, the pole piece 29 adjacentthe output wave guide 26 has an aperture there through to accommodatethe heat radiator portion 24 of the traveling wave tube 20 adjacent theelectron collector 23.

In the specific illustrative embodiment of my invention depicted in Fig.4, an eccentric unit assembly 30 is mounted coaxially on the exit end ofthe traveling wave tube 20 adjacent the heat radiator portion 24 andprovides for alignment of the tube axis with the magnetic axis. Trimmingscrews around the pole pieces 23 and 29 provide for additionalcorrection, if necessary. I have found that uniformity of the magneticfield in the interaction space is assured by shaping of the magnetassembly 10 to the configuration shown in Fig. 2, so that flux guides,required in other configurations to provide the requisite uniformity,are not required here. The tapered extension ears 13 also provide ashield for the interaction space against stray external magnetic fields.

inhomogeneities in the material of the permanent magent assembly 10 andits close proximity to the magnetic field axis may cause slight fieldcomponents transverse to the axis which disturb the uniform straightlongitudinal field set up in the interaction space. Such transversefields may be offset by additional magnetic shielding. The shieldingmeans referred to in this instance as field straighteners, may comprisespaced apart coaxial rings or discs 35, Fig. 5, of high permeabilitymaterial located between the interaction space and the magnetic wallsdefining the interaction space. Such shielding means is well known inthe art. The discs would be supported by supports 36 and 37 and spacerdiscs 38 of nonmagnet-ic material in such a manner as not to be incontact with each other, with the pole pieces 28 and 29, or with themagnetic walls defining the interaction space. A hollow mandrel 39 of aninsulating material may be inserted between the shielding discs 35 andthe interaction space through which the tube 22 extends to provideadjustable support for the discs 35 and to permit accurate alignment topromote optimum shielding. The discs used in this instance act to alignthe transverse fields normal to the magnetic axis so as not to disruptthe uniformity or the main field. Adequate shielding against externalmagnetic lields is supplied by the magnet assembly 10.

The open sections of the magnet assembly adjacent each end thereof, dueto the taper of the extension ears 13, are necessary to accommodate thewave guide cou pling arrangement for the traveling wave tube 26. Theseopen or bridging sections of the magnet assembly it) are effective toextend the magnetic field of the magnet assembly it) to include the areaoccupied by the wave guides 25 and 26, the tapered ears 13 providing thenecessary field uniformity and distribution in these areas. Thispeculiar shaping of the magnetic material is sufficient without fluxguides to realize the desired field uniformity in the coupling areas,thus permitting a material saving in materials, fabrication, size andweight.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artWithout departing from the spirit and scope of this invention.

What is claimed is:

1. An electron discharge device comprising an electrical conductordefining an elongated electromagnetic wave transmission system, an inputand an output coupling means for said transmission system, said couplingmeans being spaced apart from each other along said conductor, electrongun means adjacent one end of said conductor for projecting an electronstream lengthwise of and in coupling relation to said conductor andmeans applying a magnetic field along said conductor to focus saidelectron stream, said last-mentioned means comprising a pair of flatpermanent magnet members positioned on opposite sides of said conductor,each of said members having a pair of projecting ears extending from thesides of said members, the ears of said members being contiguous to eachother and said ears increasing both in length and in width away from thepoint of contact with each other and toward said fiat members, and saidcoupling means extending between said fiat members adjacent the ends ofsaid projecting ears.

2. An electron discharge device comprising means defining anelectromagnetic wave transmission system, electron gun means adjacentone end of said system for projecting an electron stream lengthwise ofand in coupled relationship thereto, electron receiving means adjacentthe other end of said system and means applying a magnetic field alongsaid system to focus said electron stream, said last-mentioned meanscomprising a permanent magnet member having a first and a second bedportion, a first pair of protuberances from said first bed portion, asecond pair or" protuberances from said second bed portion, said memberarranged such that each of said first protuberances is contiguous to acorresponding one of said second protuberances, said protuberancesincreasing in cross section away from the point of contact with eachother and toward said bed portions.

3. An electron discharge device in accordance with claim 2 wherein saidprotuberances extend from opposite sides of said bed portions andregularly increase in length away from said point of contact and towardopposite ends of said bed portions and increase in thickness away fromsaid point of contact and toward the centerline between said oppositesides of said bed portions.

4. An electron discharge device in accordance with claim 2 wherein saidprotubenances on opposite sides of one of said first and second bedportions are joined 'below the surface of said one of said bed portions.

5. An electron discharge device in accordance with claim 2 wherein saidprotuberances on opposite sides of one of said first and second bedportions are joined above the surface of said one of said bed portions.

6. An electron discharge device comprising an electrical conductordefining an elongated electromagnetic wave transmission system, an inputand an output coupling means for said transmission system, said couplingmeans being spaced apart from each other along said conductor, electrongun means adjacent one end of said conductor for projecting an electronstream lengthwise of and in coupling relation to said conductor andmeans applying a magnetic field along said conductor to focus saidelectron stream, said last-mentioned means compris ing a permanentmagnet member having first and second bed portions of substantiallyparallelepiped configuration positioned on opposite sides of saidconductor, a first pair of projections from opposite sides of said firstbed portion substantially in parallel planes normal to the plane of saidfirst bed portion, a second pair of projections from opposite sides ofsaid second bed portion substantially in parallel planes normal to theplane of said second bed portion and arranged such that each of saidfirst extensions is contiguous at its apex to a corresponding one ofsaid second extensions at its apex, each of said extensions increasingboth in length and in width away from its apex and toward said bedportions, and said coupling means extending between said flat membersadjacent the ends of said projecting ears.

7. An electron discharge device in accordance with claim 6 wherein saidprojections from opposite sides of one of said first and second bedportions are joined below the surface of said one of said bed portions.

8. An electron discharge device in accordance with claim 6 wherein saidprojections from opposite sides of one of said first and second bedportions are joined above the surface of said one of said bed portions.

9. An electron discharge device in accordance with claim 6 and furthercomprising means for aligning the axis of said magnet members with theaxis of said conductor.

10. An electron discharge device in accordance with claim 9 and furthercomprising shielding means positioned between said magnet member andsaid conductor and substantially encompassing said conductor.

11. An electron discharge device in accordance with claim 10 whereinsaid shielding means comprises a plurality of spaced-apart magneticdiscs disposed along the axis of said magnet member.

12. Traveling Wave tube apparatus comprising a permanent magnet memberhaving first and second fiat sections spaced apart by extensions fromopposite sides of each section, pairs of said extensions beingcontiguous at an apex and each extension sloping downwardly from theapex toward the ends of said fiat sections and toward the median of thesides of said flat sections, an elongated helical conductor extendingbetween said flat sections, and means for projecting a stream ofelectrons lengthwise of and in coupled relationship to said conductor.

13. An electron discharge device in accordance with claim 12 and furthercomprising means for aligning the axis of said magnet members with theaxis of said conductor.

14. An electron discharge device in accordance with claim 12 and furthercomprising shielding means positioned between said magnet member andsaid conductor and substantially encompassing said conductor.

15. An electron discharge device in accordance with claim 14 whereinsaid shielding means comprises a plurality of spaced-apart magneticdiscs disposed along the axis of said magnet member.

No references cited.

